See, that's where you are wrong
(At least, judging by the pictures)
What you mistook for a return line is actually a tank vent
That line should have gone into a charcoal canister which would allow air to enter the tank, but reroute air coming from the tank into the intake (while there is engine vacuum present)
Now, with that canister missing it is pretty easy to look at it and think it is a return line, but by hooking it up like you did, there is no way for air to enter the tank, as the pump pulls out gas...it seems to me you are forcing your tank into a vacuum situation